This invention relates to light reflective and light transmissive electrochromic devices. More specifically it relates to an improved electrochromic device which is characterized by an increased resistance to environmental change. A transmissive electrochromic device is one which normally transmits light, but which can be activated to become opaque, either entirely across the surface of the device or across a portion of the surface, as for example when one wishes to display a message or the like. An electrochromic reflective device is a mirror which can be darkened to diminish reflectance and thereby minimize glare.
Electrochromic devices can be made by depositing an electrochromic stack on a transparent substrate, typically glass. The electrochromic stack itself typically comprises a first transparent conductive layer deposited on the substrate, followed by sequentially deposited layers to form a stack as follows: An electrochromic layer, an ion conductive layer, and another active layer (which may either be another electrochromic layer or a conductor layer). If the second active layer is itself electrochromic, it is covered by another conductive layer. In an electrochromic mirror a reflecting layer such as aluminum, stainless steel, chrome etc. is applied, which may double as the final conductive layer. A transparent or a semitransparent conductive layer such as ITO, gold etc. is used for a transmissive device.
The electrochromic layer works by an application of voltage across the cross section of the stack that causes ions such as H.sup.+, Li.sup.+, Na.sup.+, F.sup.-, Ag.sup.+, Cs.sup.+, or OH.sup.- etc. (or a mixture of them depending on the construction materials in the device) to diffuse from the ion conductor into the active layer(s) or from one active layer to another transported via the ionic conductor. These ions then react with the active layers to form differently colored chemical compounds. When a reverse voltage is applied (in some cases even if the two conductors sandwiching the device are shorted) the whole process is reversed such that the original state is resumed. Typical means of making electrochromic mirrors by this method are described in U.S. Pat. Nos. 3,712,710; 3,844,636 and 4,465,339 and methods for making displays are disclosed in U.S. Pat. No. 3,512,941.
Electrochromic devices are sensitive to their environment as the transport properties of the ions and even the presence of some of these ions required for the electrochromic phenomena, depends upon the humidity in the environment. To prolong the life of electrochromic devices, or to render them insensitive to the surrounding environment, or to preserve their performance at temperatures above ambient, one has to encapsulate such devices.
U.S. Pat. No. 4,852,979 discloses an encapsulated electrochromic mirror comprising a shatterproof backing about the device and a hermetic seal which protects the mirror from environmental factors.
Japanese Application No. 58-91431(A) discloses a solid state electrochromic display stack, comprising a series of thin films deposited onto a glass substrate which is sealed to protect the device from the ambient environment. The exposed surfaces of the films are covered with a xylene polymer, which is itself covered with a reaction curing-type resin such as an epoxy resin to which a solid protecting plate can be adhered.
U.S. Pat. No. 4,465,339 to Baucke et al. discloses hermetically sealed electrochromic mirrors using an adhesive, such as an epoxide adhesive, applied over a stack of thin films, deposited onto a transparent glass substrate, preferably in combination with a solid glass, plastic or metal backing plate adhered to the adhesive film.
U.S. Pat. Nos. 4,392,720 to Ganguillet et al.; 4,227,779 to Bissar et al. and 4,403,831 to Amano disclose the use of adhesive resin or solder joints to seal the working elements of electrochromic display devices from the ambient environment.
However, some electrochromic device constructions require a certain amount of moisture in their structure to perform well. To address this issue so that the mirror does not dehydrate, Japanese Patent Application 58-211123(A) to Kawamura describes placing a hydrate resin layer in contact with the electrochromic layer stack. This is further encapsulated by a water impermeable layer. The hydrated layer is described as a liquid when processed which subsequently hardens. Water is incorporated in this layer either by curing in a moist environment, or exposing the layer to a moist environment after hardening, or soaking the layer in water after hardening and enclosing it in a water impermeable layer.
There are a number of deficiencies in these methods. First, large amounts of water cannot be incorporated in these layers because the materials are not naturally water absorbing. Therefore not only is their water content low, but their tendency to retain moisture is minimal. Second, if too much water is somehow forced into them (e.g. by subjecting them to steam or elevated temperatures), they may corrode the electrochromic stack due to their close proximity. Lastly, if curing of the hydrated layer is attempted in a high humidity environment, the humidity might interfere with the curing process.
Such techniques can be effective in providing some protection from the environment or retaining moisture for a limited time period due to finite diffusion of water through the organic materials. However, it would be desirable to improve upon the moisture protection of electrochromic devices or use alternative materials and designs so that moisture can be retained without any of the drawbacks mentioned earlier in describing Kawamura's invention.